With the development of information and communications technology, various wireless communication technologies are under development. Among these wireless communication technologies, a WLAN enables wireless connection to the Internet at a home or business or in specific service provision areas using a portable terminal, such as a personal digital assistant (PDA), a laptop computer, and a portable multimedia player (PMP), based on radio frequency (RF) technology.
As standards for WLAN technology, the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards are under development. IEEE 802.11a provides a transmission speed of 54 Mbps using an unlicensed band at 5 GHz. IEEE 802.11b provides a transmission speed of 11 Mbps using direct sequence spread spectrum (DSSS) at 2.4 GHz. IEEE 802.11g provides a transmission speed of 54 Mbps using orthogonal frequency division multiplexing (OFDM) at 2.4 GHz. IEEE 802.11n provides a transmission speed of 300 Mbps for two spatial streams using multiple-input multiple-output (MIMO)-OFDM. IEEE 802.11n supports a channel bandwidth up to 40 MHz, and thus provides a transmission speed of 600 Mbps.
With the proliferation of such WLANs and the diversification of applications using WLANs, there is an increasing necessity for new WLAN technology for supporting a higher throughput than a data processing speed of IEEE 802.11n. Very high throughput (VHT) WLAN technology is one of the IEEE 802.11 WLAN technologies proposed to support a data processing speed of 1 Gbps or higher. Among these WLAN technologies, IEEE 802.11ac is being developed as a standard for providing VHT in a 5 GHz or lower band, and IEEE 802.1 lad is being developed as a standard for providing VHT in a 60 GHz band.
In a system based on such a WLAN technology, a terminal operating in a power saving mode (PSM) transmits a power save (PS)-poll frame to the corresponding AP to receive data buffered in the AP. At this time, if a plurality of terminals simultaneously transmit PS-poll frames, channel access competition occurs among the terminals, and a collision occurs among the PS-poll frames. Therefore, the terminals need to maintain an awake state or repeatedly perform retransmission of a PS-poll frame which is not transmitted due to the collision until data is successfully received from the AP, and thus a large amount of power is consumed.